Public sector planning, especially in developing countries, is both a theoretical and methodological challenge that is particularly important with regard to the field of environmental sanitation. This field is dominated by a technical vision that often ignores the complexity of the real world and the human dimension influenced by its actions, as opposed to a strategic one. This paper discusses the preparation of the Brazilian National Basic Sanitation Plan (Plano Nacional de Saneamento Básico – PLANSAB) with a particular focus on its future scenario construction phase, which was primarily based on Situational Strategic Planning and prospective methods. Three scenarios were developed in a participatory process that consisted of several steps and involved various social actors, including governmental officials and authorities, informed by a team of experts in charge of the whole planning process. During the process, 10 extrinsic and intrinsic factors influencing the sector were considered, which led to the generation of a set of macro directives, strategies and targets for the expansion of access to water supply, sanitation, solid waste management and urban storm water management over the next 20 years. PLANSAB was able to overcome many of the criticisms that are typically raised with regard to traditional planning, although, as with any political process, it will not be immune from inertial factors that may create obstacles during the implementation phase.